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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel 17734, 17735, 17736
Message-Id: <20260402070801.CE926288082F@server.astronomerstelegram.org>
Date: Thu, 2 Apr 2026 03:08:01 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17734							     ATEL #17734

Title:	Einstein Probe detection of the onset of a new X-ray outburst
		of MAXI J1816-195
Author:	H. N. Yang, X. Mao (NAO, CAS), Q. C. Liu (THU), H. Sun (NAO,
		CAS) on behalf of the Einstein Probe (EP) team
Queries:	ep_ta@bao.ac.cn
Posted:	1 Apr 2026; 09:19 UT
Subjects:X-ray, Neutron Star, Pulsar

We report the detection of a new outburst from MAXI J1816-195 with the
Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP). The source
was discovered in June 2022 (Negoro et al., 2022), and has exhibited multiple
outbursts since then. Most recently, the source underwent an outburst in
2025 (ATel #17339). EP-WXT detected an onset of a new outburst on 2026
March 28, with a flux of about 2.2e-11 erg cm-2 s-1
in 0.5-4 keV. The source was not detected in the observation on March 26,
with an exposure time of 3 ks, corresponding to a flux upper limit of 9e-12
erg cm-2 s-1. The latest WXT spectrum can be fitted
with an absorbed power-law model, with a frozen absorption column density
of 2.36e22 according to previous study and a photon index of 1.3 (+0.9/-1.1).
The unabsorbed flux in 0.5-4 keV is 3.8 (+1.7/-1.4) e-10 erg cm-2
s-1. FXT follow-up observations have been arranged.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor
the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook
of X-ray and Gamma-ray Astrophysics).

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Password Certification: Yuan Liu (liuyuan@bao.ac.cn)
https://www.astronomerstelegram.org/?read=17734
==============================================================================
ATEL #17735							     ATEL #17735

Title:	VLA non-detection of a compact persistent radio source associated
		with the repeating FRB 20251229A
Author:	Gabriele Bruni (INAF-IAPS), Luigi Piro (INAF-IAPS), Aishwarya
		L. Thakur (INAF-IAPS), Luciano Nicastro (INAF-OAS), Andrea Rossi (INAF-OAS),
		Eliana Palazzi (INAF-OAS)
Queries:	gabriele.bruni@inaf.it
Posted:	1 Apr 2026; 09:42 UT
Subjects:Radio, Fast Radio Burst

We report on Karl G. Jansky Very Large Array (VLA) observations of the
repeating fast radio burst FRB 20251229A, aimed at searching for a compact
persistent radio source (PRS) associated with the burst.

The observations were carried out on March 30, 2026, in C band (4-8 GHz)
and A configuration, providing a synthesized beam of 0.38" x 0.37" at 6
GHz, corresponding to ~0.7 kpc at the redshift of the candidate host galaxy
WISE J204123.12+160126.2 (z=0.12, as computed from optical SED fitting
with CIGALE). The observations were centered on the coordinates of the
candidate host galaxy (ATel #17709, ATel #17723). The total on-source integration
time was approximately 1 hour within a 1.5 hour observing block, reaching
an rms sensitivity of ~3.3 uJy/beam.

No compact radio source is detected at the position of the candidate PRS
reported in ATel #17723, down to a 3-sigma upper limit of 10 uJy at 6 GHz.
This corresponds to an upper limit on the isotropic spectral luminosity
of L<3.6x10^27 erg/s/Hz, assuming a standard cosmology.

Recent uGMRT observations reported a detection of S~64 uJy in Band 4 (550-750
MHz, ATel #17723). Combining this measurement with our VLA upper limit
implies a spectral index alpha<-0.8, indicating a steep spectrum between
0.65 and 6 GHz. It is worth noting that the uGMRT Band 4 beam (~4-6") encompasses
the entire host galaxy (~10 kpc at z=0.12), so the detected 650 MHz emission
may include a significant contribution from integrated star formation;
in contrast, our VLA A-configuration observations at 6 GHz resolve the
host on sub-kpc scales (~0.7 kpc), isolating compact emission at the putative
PRS position. While a contribution from a compact persistent source at
low frequencies cannot be excluded, higher-resolution observations are
essential to disentangle the origin of the emission.

The achieved sensitivity in these VLA observations is sufficient to detect
compact persistent sources comparable to those associated with several
repeating FRBs when scaled to this distance. The non-detection therefore
places stringent constraints on the presence of a compact synchrotron nebula
powered by the FRB central engine.

The National Radio Astronomy Observatory and Green Bank Observatory are
facilities of the U.S. National Science Foundation operated under cooperative
agreement by Associated Universities, Inc.

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Password Certification: Gabriele Bruni (gabriele.bruni@inaf.it)
https://www.astronomerstelegram.org/?read=17735
==============================================================================
ATEL #17736							     ATEL #17736

Title:	Discovery of a Probable Nova in M81, M81N 2026-04a
Author:	K. Hornoch, P. Fatka, H. Kucakova (Ondrejov), A. W. Shafter
		(SDSU)
Queries:	k.hornoch@centrum.cz
Posted:	2 Apr 2026; 00:27 UT
Subjects:Optical, Nova, Transient

We report the discovery of a probable nova in M81 on a co-added 3720-s
unfiltered CCD frame taken on 2026 Apr. 1.893 UT with the 0.65-m telescope
at Ondrejov under poor conditions (strong moonlight and cirrus clouds).

The object designated as M81N 2026-04a = PNV J09553883+6905217 is located
at R.A. = 9h55m38s.83, Decl. = +69o05'21".7 (equinox 2000.0), which is
30.3" east and 86.6" north of the center of M81 (see link to discovery
image below).

Here we list the observing dates and corresponding photometry:

Date [UT] | Mag | Err | Filter | Telescope

2026-03-29.918 | >21.4  |      | C       | OND 
2026-04-01.893 |  19.4  | 0.15 | C       | OND

As with all extragalactic novae, follow-up spectroscopic and photometric
observations to determine the nova's spectroscopic class and rate of decline
(speed class) are encouraged. The OND 0.65-m is a reflecting telescope
at the Ondrejov observatory operated jointly by the Astronomical Institute
of ASCR and the Astronomical Institute of the Charles University of Prague,
Czech Republic. It uses a Moravian Instruments G2-3200 MkII CCD camera
(with a Kodak KAF-3200ME sensor and standard BVRI photometric filters)
mounted at the prime focus. The unfiltered OND photometry was calibrated
against R-band comparison stars from 
<a href="http://adsabs.harvard.edu/abs/1995AJ....109.1055P">Perelmuter
& Racine (1995)</a>.

Discovery image: http://www.asu.cas.cz/~asteroid/M81N2026-04a.png
------------------------------------------------------------------------------
Password Certification: Allen W. Shafter (ashafter@sdsu.edu)
https://www.astronomerstelegram.org/?read=17736
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